This invention relates to a press molding apparatus and a press molding method for use in precision pressing in which an optical element is obtained by press molding a material, such as a preformed glass material in a heated and softened state and, in particular, to precision pressing in which no polishing is required after molding.
Recently, in a field of production of an optical element such as an optical lens, it is desired to obtain a high-accuracy lens shape without carrying out surface polishing. To this end, proposal is made of a method comprising the steps of preparing a pre-shaped glass material (preform), heating and softening the preform, and pressing the preform by a high-accuracy pressing surface (Japanese Unexamined Patent Publication JP 2001-10829 A). In particular, use is recently made of a mother mold having an elongated shape with a plurality of molding surfaces arranged in a single line so as to simultaneously press form a plurality of preforms (Japanese Unexamined Patent Publication JP 11-29333).
In the meanwhile, upon producing the optical element by precision pressing, accuracy and productivity are important aspects.
In this sense, anisothermal pressing (Japanese Unexamined Patent Publication JP 8-133756 A) has contributed to epoch-making progress. Specifically, by shortening a heating cycle of the mother mold as compared with existing isothermal pressing, the cycle time required to form the glass optical element can be shortened to the order of several tens of seconds. In addition, surface accuracy and profile accuracy can be kept superior.
Taking the production efficiency into account, attention is directed to a method of obtaining a plurality of optical elements in one heating cycle, i.e., a multi-product batch process. As far as the heating cycle is essential and requires a predetermined time period, the productivity can be improved if a plurality of optical elements are simultaneously produced in the heating cycle.
In the meanwhile, one of design options for the mother mold capable of simultaneously producing a plurality of optical elements is to dispose a plurality of molding surfaces in a single-line arrangement (JP 11-29333 A mentioned above). Such single-line arrangement is advantageous in the following respects. That is, the structure of the mother mold is simple. In particular, consideration will be made of supply of the glass materials to the mother mold. In the state where the glass materials are arranged in a single line, a supplying member is split by a straight line into two parts to drop the glass materials through a gap between the two parts. With such a simple mechanism, the glass materials are simultaneously supplied onto the mother mold (i.e., to the respective molding surfaces).
In order to drop the glass material in a heated and softened state, the glass material in the softened state is floated on a floating saucer by the use of a gas and then dropped and supplied to the mother mold. This technique is advantageous in that the glass material is stably supplied without damaging the surface of the glass material. For example, by arranging a plurality of floating saucers in a single line and splitting each floating saucer into two parts, the glass materials are simultaneously dropped through gaps between the two parts onto the molding surfaces arranged in a single line. In this case, the apparatus is relatively simple in structure. Thereafter, press molding can be immediately performed before the temperature of the glass material is changed from the preheat temperature. The above-mentioned technique is very advantageous in that the productivity is high and a plurality of optical elements can be stably and accurately produced under a thermally uniform condition.
As described above, the linear arrangement of the molding surfaces on the mother mold is advantageous. However, such design of the mother mold is disadvantageous in the following respects.
If the mother mold having an elongated shape is provided with a plurality of molding surfaces arranged in a single line and if the mother mold is warped or deformed due to temperature difference in its thickness direction, upper and lower molding surfaces are inclined. The effect of such inclination is greater towards opposite ends of the mother mold in its longitudinal direction. This results in occurrence of tilt in molded products, such as optical lenses, and thickness deviation of the formed products. The tilt and the thickness deviation are greater in those products formed by the molding surfaces nearer to the opposite ends of the mother mold. Recently, the problem of warp of the mother mold becomes more and more serious. This is because, following the recent demand for reduction in cycle time of the press molding process, rapid heating and rapid cooling are carried out. In addition, following the recent demand for simultaneous production of a greater number of products, the mother mold is further elongated.
One of the main factors causing the warp is a temperature gradient produced in the mother mold, in particular, a temperature gradient in the vertical direction. In case where the molding surfaces are arranged in series in a single line, the tilt and the thickness deviation become more and more serious under a greater influence of warp of the mother mold as a greater number of molding surfaces are arranged on the mother mold and as the molding surfaces are arranged nearer to the opposite ends of the mother mold.
For example, FIG. 1 shows a basic structure of a typical press molding apparatus of the type mentioned above. The press molding apparatus illustrated in FIG. 1 has a pressing mold comprising an upper mother mold 502 and a lower mother mold 504. Each of the upper and the lower mother molds 502 and 504 has an elongated shape extending in a transversal or horizontal direction in the figure. The upper and the lower mother molds 502 and 504 are supported by upper and lower supporting members 506 and 508, respectively. The upper supporting member 506 is attached to a fixed shaft 510 while the lower supporting member 508 is attached to a drive shaft 512 of a motor mechanism or the like. The upper and the lower mother molds 502 and 504 have a plurality of molding portions 514 and 516 formed on confronting surfaces thereof, respectively, to provide preforms with a lens shape. To a position between the upper and the lower mother molds 502 and 504, the preforms each of which is preliminarily formed into a desired provisional shape are transferred after heated by a heating unit (not shown) to a predetermined temperature, for example, to a temperature corresponding to a viscosity between 105.6 and 109 poises. The upper and the lower mother molds 502 and 504 are surrounded by induction heating coils 518 and 520 for heating the upper and the lower mother molds 502 and 504, respectively. The upper and the lower mother molds 502 and 504, which are preliminarily heated, clamp and press the preforms in a softened state to thereby form high-accuracy processed surfaces on the preforms.
Herein, temperature distribution is caused in the upper and the lower mother molds 502 and 504 in their thickness directions. This is because the heat of the upper and the lower mother molds 502 and 504 is dissipated through the supporting members 506 and 508, respectively. Such temperature difference may possibly results in occurrence of the warp in the upper and the lower mother molds 502 and 504 as schematically depicted by dot-dot-dash lines in FIG. 1. If press molding is performed in the state where the upper and the lower molds are warped, defective molding may be caused and the upper or the lower mother mold 502 or 504 may be damaged. If the degree of parallelism is decreased due to the warp, upper and lower surfaces of press formed products will be inclined. If the required product specification is strict, a desired performance may not be achieved.
In particular, it is recently proposed to further increase the lengths of the upper and the lower mother molds 502 and 504 so as to simultaneously form a greater number of (for example, six) preforms for the purpose of improving the production efficiency and to form lenses having greater diameters. Under the circumstances, it is an urgent demand to suppress the warp.